1. Field of the Invention
The present invention relates to a power-transmission control mechanism of a lawn mower wherein the power of an internal combustion engine is transmitted to a mowing blade through an electromagnetic clutch.
2. Description of the Related Art
A mechanical clutch such as a friction clutch is used in lawn mowers in order to control the transmission of power from the internal combustion engine to the mowing blades. Rapid load fluctuation in the internal combustion engine, which is caused by the load such as inertia of the blades, has been prevented by gradually engaging the clutch (going through a half clutch state). Heretofore, running stop (engine stall) of the internal combustion engine under the load has thus been prevented.
In a lawn mower using an electromagnetic clutch in its power transmission mechanism (for example, refer to JP-A-9-36), power is transmitted through the engagement between rotating disks. Thus, in this type of electromagnetic clutch, it is not possible to avoid rapid load fluctuation because there are no damping elements, and therefore engine stall tends to occur when load is applied.
In general, there has been known a method of pulse-driving the electromagnetic clutch in lawn mowers in which a large load fluctuation is apt to be imposed to the internal combustion engine when the electromagnetic clutch is engaged (for example, refer to JP-A-58-191326).
The power transmission control disclosed in JP-A-58-191326 is a control for pulse-driving the electromagnetic clutch by gradually increasing the pulse width.
A complex control circuit is required to pulse-drive the electromagnetic clutch while the pulse width is gradually increased. Further, load following characteristic of the internal combustion engine, the characteristic of the clutch, and load fluctuation need to be considered to set the pulse width. For this reason, it is not easy to set the pulse width. Even though the pulse width is set once, the set pulse width cannot cope with characteristic changes with time of the clutch.
In addition, when the electromagnetic clutch is pulse-driven, abrasion of the electromagnetic clutch is expedited because the clutch is maintained under a “half clutch” condition.